FIG. 1 shows a known burner. This burner has a cone shaped chamber 1 defined by two part cone shells 2 wherein air 3 can be introduced through slots 4.
The air generates in the centre of the cone shaped chamber 1 (i.e. along the axis 5 of the cone shaped chamber 1) a zone of larger vortices 6 (the vortex core).
A lance 8 is provided along the axis 5 to inject a thin liquid fuel jet 15 into the cone shaped chamber 1. In particular the liquid fuel jet 15 can be injected into the vortex core 6 to mix with the air and form a combustible mixture.
Nevertheless, when the liquid fuel jet cross-section is too small, it withstands large asymmetrical centrifugal forces because the liquid fuel jet can not reliably stay within the equally small vortex core and misses the centre, with large gradients of circumferential velocity, which then can prevent it from staying at the vortex core. In practice, during operation the liquid fuel jet 15 fluctuates radially around the vortex core.
These fluctuations can lead to combustion instabilities that are amplified in the burner and combustion chamber downstream of the burner.
U.S. Pat. No. 6,270,338 describes a burner of a gas turbine having these features.
Combustion instabilities can influence both the lifetime and noise emissions. In particular, low frequency instabilities with a frequency less than 30 Hz can be difficult to deal with.
In fact, it can be difficult to suppress these instabilities with operation changes, and damping of these low frequency's instabilities using, for example, Helmholtz dampers can be difficult, because of the huge resonator volumes that would be required.
These problems can also be increased by the fact that low frequency pulsations couple the exhaust system, amplify the noise and propagate it into the neighbouring areas of the power plant.
Burners having a lance with a divergent outlet are also known.
In this respect, WO 03/054447 discloses a lance having a tip with a diverging portion and a diverter facing it. The diffuser angle is very large and also thanks to the diverter, the fuel jet can be diverted laterally generating a conical fuel flow.
U.S. Patent Application No. 2003/150217 discloses a lance with a large conical tip arranged to fan out the fuel after injection.
DE 19537636 discloses a lance with a very short diverging portion with a wide diverging angle. This diverging portion can be arranged to generate a conical fuel flow.
EP 692675 and DE 4446609 disclose a lance having a cylindrical end that feeds the fuel in a conical atomisation chamber wherein atomisation air is injected. The mixture formed in the atomisation chamber can then be fed to a conical burner chamber. In these burners the lance does not inject a liquid jet (in the form of a liquid cylinder) into the vortex core.